Many state of the art computers and other electronic products of today employ numerous printed circuit boards. Load current from a current supply is delivered to the printed circuit boards through complex and costly buswork. In order to reduce this buswork, it is desired to mount power converting circuits on the individual circuit boards so that the power can be distributed at a higher voltage (e.g. 42 volts) and then converted at the point of load on each board to the logic level voltage of about 5 volts. However, since each board has such limited area and tight spacing, these point-of-load converters should have a power handling density of at least 50 watts/in.sup.3 in order to make such a distributive power system viable.
However, the largest density typical of today's power electronic technology is about 5 watts/in.sup.3. In order to achieve the desired improvement in this density, the switching frequency of power circuits has been significantly increased above the common 100 kHz range. This increase reduces the size of the transformers and filter elements that currently occupy a large portion of the circuits overall volume. Recent commercial efforts in the 1 MHz range have shown the potential for this approach, but their densities (around 15 watts/in.sup.3) are still too low. Thus, an increase in switching frequency to the 5-10 MHz range is necessary for the desired distributed power system.
The switching of a power circuit in the 10 MHz range is disclosed by:
Nathan O. Sokal and Alan O. Sokal "Class E-A New Class of High Efficiency Tuned Single-ended Switching Power Amplifiers", IEEE Journal of Solid-State Circuits, Vol. sc-10, No. 3, June 1975;
Ronald J. Gutmann, "Application of RF Circuit Design Principles to Distribute Power Converters", IEEE Transactions on Industrial Electronics and Control Instrumentation, Vol. IECI -27, No. 3, August 1980; and
Andrew F. Goldberg and John G. Kassakian, "The Application of Power MOSFET's at 10 MHz," PESC '85--Proceedings of the 16th Annual IEEE Power Electronics Specialists Conference, Toulouse, France, June 24-28, 1985, pages 91-100.
The disclosed devices and techniques make it evident that a feature known as resonant switching is necessary in order to achieve such a high switching frequency. A "resonant switch" is generally a subcircuit having a semiconductor controllable switch (i.e. a MOSFET) connected in series with an inductor and a capacitor connected in parallel with either the semiconductor switch or the combined series connection of the inductor and the semiconductor switch. The inductor and capacitor constitute a resonant LC circuit whose oscillation is initiated by either the turn-off or turn-on transition of the switch, and whose resonance is used to shape either the voltage or the current waveform of the switch.
Another disclosed feature of the topology of a 10 MHz converter is that the topology must not call for a step change in the voltage across a semiconductor device (i.e., the MOSFET switch). That is, even though the MOSFET is a majority carrier device, the energy stored in its junction capacitance is too great to lose every cycle at a 10 MHz rate.
One topology that seems to encompass the foregoing features is the "zero-voltage quasi-resonant converter" disclosed by
K. Liu, F. C. Lee in "Zero Voltage Switching Technique in DC/DC Converters", IEEE PESC 1986 Record, pages 58-70;
K. Liu, F. C. Lee, "Resonant Switches-A Unified Approach to Improved Performance of Switching Converters", 1984 IEEE International Telecommunications Energy Conference Proceedings, pages 334-341;
K. Liu, R. Oruganti, F. C. Lee, "Resonant Switches-Topologies and Characteristics", IEEE PESC 1985 Record, pages 106-116.
These converters can be thought of either as a modification of a square wave topology or as half of a "classical" resonant converter (with a rectified load) operated in a discontinuous mode. These converters place a resonant capacitor in parallel with the controllable switch so that its voltage will rise slowly from zero at the turn-off transition and ring sinusoidally back to zero in time for the turn-on transition. This position of the capacitor is consistent with the switches own parasitic capacitance, and at a high enough frequency a separate element is not needed. By rearranging the relative positions of the switches, the resonant elements, and the input/output filter elements, topological forms that correspond to each of the various dc-dc converters can be generated.
A first problem with the zero voltage quasi resonant converter is its dependence on load. Under light load, the resonant ring of voltage across the controlled switch does not have sufficient amplitude to return to zero before the switch is again turned on. The energy that remains in the resonant capacitor at this instance in the cycle will therefore be lost. To avoid this dissipation, the characteristic impedance of the circuit could be made larger, but doing so would increase both the energy storage requirements of the resonant elements and the voltage to which the controlled switch is stressed at high load.
A second problem with this converter is that only the controlled switch has zero voltage transitions. The output or rectifying diode on the secondary side of a transformer does not. There are therefore losses associated with the rectifier's junction capacitance at both the turn-on and turn-off transitions. The importance of these losses depends on the relative size of this capacitance compared to that in parallel with the MOSFET (the resonant controllable switch). Since the initial purpose of a power converter discussed above requires a stepdown transformer, the turns ratio helps to make the size comparison between the capacitance and the MOSFET favorable, but the need to provide a large area diode, or even a sychronous rectifier, for low conduction loss makes this comparison unfavorable.